(1) Field of the Invention
The present invention relates to a high intensity discharge lamp for use in such apparatuses as general illumination apparatuses and vehicle headlamps that use the lamp mounted to a reflector, a driving apparatus for a high intensity discharge lamp, and a high intensity discharge lamp system employing the lamp and the driving apparatus.
(2) Description of the Related Art
In recent years, high intensity discharge lamps have been employed for projectors and vehicle headlamps in which the lamps are mounted to reflectors. Such discharge lamps exhibit high efficiency, low power consumption, and high intensity in comparison with halogen lamps, and therefore are expected to receive more widespread commercial acceptance.
When a conventional high intensity discharge lamp is positioned in such a manner that the line connecting the pair of electrodes becomes horizontal, and operated by applying voltage across the pair of electrodes, (the operation in this manner is hereinafter referred to as xe2x80x9chorizontal operationxe2x80x9d or xe2x80x9chorizontally operatedxe2x80x9d), the arc is bent upwards. As a result, the temperature of the upper portion of the arc tube locally becomes high, which causes such problems that devitrification of the upper portion of the arc tube or deformation of the arc tube begins at a relatively early stage in lamp life and thus the lamp life reduces.
In view of the problems, much research has been carried out to develop the technique for suppressing the arc bend and thereby improving lamp life. For example, Japanese Unpublished Patent Publication Nos. 55-86062 and 9-161725 disclose a technique for suppressing the arc bend by applying a magnetic field to a metal halide lamp.
The technique disclosed in Japanese Unpublished Patent Publication No. 55-86062 is such that by providing a strong rare earth magnet above the arc tube to apply a magnetic field to the arc tube from the upper direction thereof, the arc is forced downwards by the repulsion force between the magnet and the arc so that the upward arc bend is suppressed.
Recently, as environmental concerns have increasingly been regarded as important, it has been required that mercury not be used for general high intensity discharge lamps. However, the lamps disclosed in the above publications employ mercury as a filling material to be enclosed in the arc tube. Therefore, in order to meet such requirements as described above, it is necessary that the arc bend be suppressed in the high intensity discharge lamp in which mercury is not employed to improve the lamp life.
In addition, the high intensity discharge lamp requires a dedicated operating circuit, which incurs higher cost. Unlike a halogen lamp having two filaments, it is difficult for the high intensity discharge lamp to attain both high beam and low beam with only one lamp. For this reason, in a vehicle headlamp system, normally, two high intensity discharge lamps are used for low beam lamps in which the frequency of use is high, and two halogen lamps are used for high beam lamps. Thus, this configuration also requires four sets of mechanism for adjusting the reflector and the light axis, which also incurs high manufacturing cost and limits the freedom in the vehicle designing.
In view of this problem, there is a need for a technique by which high beam and low beam can be selected with the use of only one high intensity discharge lamp. A typical example of such a technique is, for example, described in Japanese Unexamined Patent Publication No. 11-312495, in which by using an electromagnet, the arc bend amount is varied to move the position of the light source. This technique is such that, in a discharge lamp disposed in a reflector such that the axis line of the discharge electrodes is in a horizontal direction (a horizontally operated lamp), a magnetic field transverse to the axis line is applied to the arc by an electromagnet. By applying a magnetic field in such a manner, the arc can be deflected (bent) in a downward direction by the effect of Lorentz force. In addition, by controlling the electric power supplied to the electromagnet, the Lorentz force can be varied such that selecting of low beam and high beam is possible.
However, according to a technique of controlling arc deflection by an electromagnet such as described above, an additional circuit for controlling the electric power supplied to the electromagnet is necessary, which incurs complicated device configurations and the increase in manufacturing cost. It might be possible to control the arc deflection by varying a lamp current or a discharge electrode gap, rather than varying the electric power supplied to the electromagnet, but in practice, it would be very difficult to control variations in such factors.
In addition, the present inventors found in the course of various experiments to accomplish the invention that when a magnetic field is applied to a discharge lamp, variation of luminance, flickering, occurs at times. This flickering can be classified into two kinds: the flickering that occurs at the start of the light beam immediately after the start of the lamp and the flickering that occurs during the stable operation
The present invention has been accomplished in view of the foregoing and other problems in prior art. Accordingly, it is an object of the present invention to provide a mercury-free high intensity discharge lamp in which mercury is not contained inside the arc tube, and arc bend is suppressed and long lamp life can be obtained.
It is another object of the invention to provide, with a simple and low cost configuration, a high intensity discharge lamp in which at least one of an arc bend amount and an apparent arc width can be controlled.
It is further another object of the invention to provide a driving apparatus for the high intensity discharge lamp.
It is still another object of the invention to provide a high intensity discharge lamp system employing the high intensity discharge lamp and the high intensity discharge lamp system.
It is to be understood that the aspects of the present invention to be detailed hereinafter are accomplished on the basis of an identical technical idea. Nonetheless, each of the aspects is embodied by a different embodiment or an example, and accordingly, the invention is divided into two groups, Embodiment I and Embodiment II, in which more closely related aspects are grouped. Hereinafter, the details are discussed according to each embodiment.
In the process of the research and development of a high intensity discharge lamp that does not use mercury, the present inventors found that the high intensity discharge lamp in which mercury is enclosed in the arc tube exhibits a larger degree of arc bend than a discharge lamp in which mercury is enclosed in the arc tube.
When the present inventors performed the experiment in which a magnetic field is applied to the high intensity discharge lamp having a large degree of arc bend to suppress the arc bend, the present inventors surprisingly found that the magnetic flux required to eliminate the arc bend is smaller in the lamp in which mercury is not enclosed than in the lamp in which mercury is enclosed, that is, the lamp in which mercury is not enclosed does not require a strong magnetic field. The present inventors considered that the principle of suppressing the arc bend in the high intensity discharge lamp in which mercury is not enclosed may differ from that in the lamp in which mercury is enclosed.
This phenomenon was discovered from the study of the degree of the arc bend in which a magnetic field is applied to each of the lamp in which mercury is not enclosed in the arc tube and the lamp in which mercury is enclosed in the course of the research and development. By the discovery of such a phenomenon, a mercury-free high intensity discharge lamp of the invention, which utilizes a low-cost and practical technique to suppress the arc bend and thereby to improve lamp life, has been accomplished.
Thus, the foregoing and other objects are accomplished, in accordance with a first aspect of the invention, by the provision of a mercury-free high intensity discharge lamp comprising:
an arc tube;
a pair of electrodes provided in the arc tube so as to be opposed to each other; and
means for generating a magnetic field having a magnetic flux component along a direction perpendicular to a line connecting a tip of one of the electrodes to a tip of the other electrode.
Further, in accordance with a second aspect of the invention, there is provided a high intensity discharge lamp as set forth in the first aspect of the invention, further comprising:
means for generating alternating current to be applied across the pair of electrodes; and
wherein the lamp is operated with the alternating current supplied from the means for generating alternating current.
By employing these configurations, in the case of the high intensity discharge lamp in which mercury is not enclosed in the arc tube, the arc bend can be suppressed by the means for generating a magnetic field by applying a magnetic field smaller than in the case of the lamp in which mercury is enclosed in the arc tube, and thereby the improvement in lamp life of a high intensity discharge lamp can be achieved in a low-cost and practical manner. In addition, by employing the alternating current operation, the arc bend caused by the Lorentz force acting on the arc in one direction can be prevented.
In accordance with a third aspect of the invention, there is provided a high intensity discharge lamp as set forth in the first aspect of the invention, wherein the lamp is disposed such that the line connecting the tips of the electrodes is horizontal, and the means for generating a magnetic field applies a magnetic field in a vertical direction.
When the lamp is disposed such that the line connecting the tips of the electrodes is horizontal as described above, an arc bent upwards tends to be formed, and therefore by applying the magnetic field in a vertical direction with the means for generating a magnetic field during the operation, the suppressing of the arc bend can be easily attained.
In accordance with a fourth aspect of the invention, there is provided a high intensity discharge lamp as set forth in the first aspect of the invention, wherein the means for generating a magnetic field is a permanent magnet.
When the high intensity discharge lamp is configured such that a constant magnetic field is applied with a permanent magnet, the suppressing of the arc bend and the longer lamp life can be achieved at low cost.
In accordance with a fifth aspect of the invention, there is provided a high intensity discharge lamp as set forth in the first aspect of the invention, wherein a metal halide is enclosed in the arc tube, and a vapor pressure of the metal halide at a temperature of 900xc2x0 C. is 0.1 MPa or higher. In accordance with a sixth aspect of the invention, the metal halide of the six aspect of the invention may include indium halide. The metal halide may include InI or InI3.
By employing such a configuration in which a metal halide is enclosed in the arc tube, the density of the metal halide is increased in the arc tube and thereby the arc becomes thinner, which reduces the width of the arc generated between the pair of the electrodes. When the width of the arc is reduced, the arc tends to become more susceptible to the convection current in the arc tube, resulting in a larger arc bend. By applying a magnetic field to such an arc having a large bend, applying a magnetic field becomes more effective. As the metal halide, halides of indium are preferable, and InI or InI3 is more preferable since they cause a higher luminous efficacy.
In accordance with a seventh aspect of the invention, there is provided a high intensity discharge lamp as set forth in the first aspect of the invention, wherein the means for generating a magnetic field is a film composed of a magnetic material formed either on a surface of the arc tube or on an outer tube provided outside the arc tube. In accordance with an eighth aspect of the invention, the means for generating a magnetic field may be supported by a supporting means. In accordance with a ninth aspect of the invention, the supporting means is a wiring member having an electrical continuity with one of the electrodes or a supporting member supporting the wiring member.
By employing these configurations, the arc bend is suppressed and thereby the lamp life of the high intensity discharge lamp can be improved.
In the course of various attempts at improving the lamp life of a high intensity discharge lamp by suppressing the arc bend, the present inventors also discovered, surprisingly, that by varying the operating frequency of the lamp current while a magnetic field is applied to the lamp, the arc bend amount, i.e., the position of the arc can be varied even at a constant magnetic intensity.
Generally, when a lamp is operated with alternating current at a predetermined frequency without applying a magnetic field, the arc is bent by the effect of the convection current inside the arc tube. When a magnetic field having a magnetic flux along the same direction of the arc bend is applied, a force that suppresses the arc bend acts on the arc, reducing the arc bend amount. When the frequency of the alternating current is further increased, the force caused by the magnetic field, which suppresses the arc bend, acts on the arc exceedingly, conversely increasing the arc bend amount. In this phenomenon, the direction of the change in the arc bend amount is the same direction as that of the magnetic flux, which indicates this phenomenon is caused by a different principle from that of the phenomenon in which, by the effect of Lorentz force, the arc bend amount changes in a direction perpendicular to the direction of the magnetic flux.
The term xe2x80x9cthe arc bend amountxe2x80x9d herein is defined to be, as shown in FIG. 14, a distance from the axis P of the electrodes 122a and 122b to the center 192 of the arc 191 when a high intensity discharge lamp is horizontally operated with, for example, a rectangular wave alternating current.
Further, the present inventors also discovered that by varying the operating frequency while a magnetic field is applied in the same direction as that of the arc bend, an apparent arc width can be varied. This phenomenon occurs because the arc receives a Lorentz force in a direction perpendicular to the arc bend, and the arc bend is shifted in the direction of the Lorentz force. When a discharge lamp is operated with alternating current, at the same time as the current polarity reversal, the direction of the Lorentz force changes into the opposite direction to the direction in which the Lorentz force has acted before the polarity reversal, and the arc is shifted in the reversed direction. Therefore, since the cycle of the electric current flowing in one direction can be varied by varying the frequency, the time in which the Lorentz force acts in one direction can be varied accordingly. For example, when the operating frequency becomes lower, the time during which the Lorentz force acts on the arc in one direction increases and the migration distance of the arc in the horizontal plane accordingly increases, making the observer perceive that the apparent arc width in the horizontal plane has increased.
The term xe2x80x9capparent arc widthxe2x80x9d herein is defined as follows. As shown in FIG. 14, a high intensity discharge lamp is horizontally operated with, for example, a rectangular wave alternating current, and, while the arc is being viewed from the top, the distance between two points 101a and 101b positioned on the line X perpendicular the line connecting the electrodes 122a and 122b at each of which points the luminance is 20% of the maximum luminance, is obtained. The distance thus obtained is defined to be the apparent arc width herein.
By the discovery of the above-described phenomenon, an aspect of the present invention has been accomplished in which at least one of an arc bend amount and an apparent arc width can be easily controlled.
In accordance with a tenth aspect of the invention, there is provided a driving apparatus for driving a high intensity discharge lamp comprising an arc tube, a pair of electrodes, means for generating a magnetic field comprising a magnetic flux component along a direction perpendicular to a line connecting a tip of one of the electrodes to a tip of the other electrode, the driving apparatus comprising:
means for generating alternating current to be applied across the pair of electrodes; and
a frequency controlling means for controlling the means for generating alternating current such that a frequency of the alternating current is varied.
In the case of employing the configuration above in which a constant magnetic field is applied as well, by varying the frequency of the lamp current, at least one of the arc bend amount and the apparent arc width can be controlled, which makes it possible to achieve a much more simplified configuration than in the case of employing an electromagnet and an electric power controlling circuit therefor. Furthermore, by employing a permanent magnet as a means for generating a magnetic field, a further simplified configuration can be achieved. It is noted that the term xe2x80x9chigh intensity discharge lampxe2x80x9d is intended to include a high intensity discharge lamp in which mercury is enclosed in the arc tube and a high intensity discharge lamp in which mercury is not enclosed in the arc tube such as described in the Summary of Embodiment I.
In accordance with an 11th aspect of the invention, there is provided a driving apparatus for a high intensity discharge lamp as set forth in the 10th aspect of the invention, wherein the frequency controlling means controls the means for generating alternating current in such a manner that the frequency of the alternating current varies with a predetermined cycle. In accordance with a 12th aspect of the invention, there is provided a high intensity discharge lamp as set forth in the 10th aspect of the invention, wherein the means for generating alternating current varies the frequency by frequency modulation, the means for generating alternating current being controlled by the frequency controlling means.
By varying the frequency, when flickering (variation in luminance) tends to occur where a magnetic field is applied to the high intensity discharge lamp, such flickering can be easily suppressed.
In accordance with a 13th aspect of the invention, there is provided a driving apparatus for a high intensity discharge lamp as set forth in the 10th aspect of the invention, wherein the frequency controlling means controls the means for generating alternating current in such a manner that the frequency of the alternating current is varied according to a lamp current.
In accordance with a 14th aspect of the invention, there is provided a driving apparatus for a high intensity discharge lamp as set forth in the 13th aspect of the invention, further comprising: means for detecting a lamp current; and
wherein the frequency controlling means controls the means for generating alternating current in such a manner that, in response to an output from the means for detecting a lamp current, the frequency of the alternating current is varied according to a lamp current.
In accordance with a 15th aspect of the invention, there is provided a driving apparatus for a high intensity discharge lamp as set forth in the 13th aspect of the invention, wherein:
the means for generating alternating current generates a larger current at the start of the high intensity discharge lamp than during a stable operation of the high intensity discharge lamp;
the driving apparatus has a timer for detecting a time at which a predetermined time from a starting of the high intensity discharge lamp has elapsed; and
the frequency controlling means controls the means for generating alternating current in such a manner that the frequency of the alternating current from the start of the high intensity discharge lamp until the predetermined time has elapsed is higher than that during the stable operation of the high intensity discharge lamp.
By employing such a configuration, the flickering, which especially occurs when a lamp current is large, can be easily suppressed in a reliable manner.
In accordance with a 16th aspect of the invention, there is provided a driving apparatus for a high intensity discharge lamp as set forth in the 10th aspect of the invention, wherein the means for generating alternating current generates an alternating current containing a ripple having a ripple rate of 10% or more
By employing alternating current containing a ripple, it is also possible to easily suppress the flickering.
Further, in the driving apparatus for the high intensity discharge lamp described above, the alternating current generated by the means for generating alternating current may be a rectangular wave current.
Thereby, the frequency of the alternating current can be easily varied and controlling of the arc bend amount can be easily performed.
In accordance with a 17th aspect of the invention, there is provided a driving apparatus for a high intensity discharge lamp as set forth in the 10th aspect of the invention, wherein the frequency controlling means changes the frequency of the alternating current supplied from the means for generating alternating current in such a manner that at least one of an arc bend amount and an apparent arc width is controlled.
By employing such a configuration, since at least one of an arc bend amount and an apparent arc width can be controlled by the frequency controlling means, when, for example, the high intensity discharge lamp is mounted to a reflector, light distribution characteristics of a light reflected from the reflector can be altered.
In accordance with an 18th aspect of the invention, there is provided a high intensity discharge lamp system comprising:
a high intensity discharge lamp comprising an arc tube, a pair of electrodes, means for generating a magnetic field comprising a magnetic flux component along a direction perpendicular to a line connecting a tip of one of the electrodes to a tip of the other electrode; and
the driving apparatus for driving a high intensity discharge lamp according to claim 10.
In accordance with a 19th aspect of the invention, there is provided a high intensity discharge lamp system as set forth in the 18th aspect of the invention, wherein the means for generating a magnetic field is a permanent magnet.
In accordance with a 20th aspect of the invention, there is provided a high intensity discharge lamp system as set forth in the 18th aspect of the invention, wherein the high intensity discharge lamp is operated such that the magnetic flux component is vertical.
In accordance with a 21st aspect of the invention, there is provided a high intensity discharge lamp system as set forth in the 18th aspect of the invention, wherein the high intensity discharge lamp is a mercury-free discharge lamp comprising in the arc tube at least a rare gas and a metal halide.
In accordance with a 22nd aspect of the invention, there is provided a high intensity discharge lamp system as set forth in the 21st aspect of the invention, wherein the metal halide comprises indium halide.
By employing the above-described configurations, by not varying the intensity of the magnetic field but varying the frequency, the arc bend amount can be easily controlled with a simple configuration, which achieves a high intensity discharge lamp system in which the configuration is further simplified with the use of a permanent magnet. In particular, when mercury is not included in the filling material in the arc tube, the controlling of the arc bend amount is further facilitated
In accordance with a 23rd aspect of the invention, there is provided a high intensity discharge lamp system as set forth in the 18th aspect of the invention, further comprising:
a reflector reflecting a light emitted from the high intensity discharge lamp; and
wherein the frequency controlling means varies a light distribution characteristic of the light reflected from the reflector by adjusting the frequency of the alternating current.
In accordance with a 24th aspect of the invention, there is provided a high intensity discharge lamp system as set forth in the 23rd aspect of the invention, wherein the frequency controlling means adjusts a direction of an optical axis of the light reflected from the reflector by changing the frequency of the alternating current.
In accordance with a 25th aspect of the invention, there is provided a high intensity discharge lamp system as set forth in the 23rd aspect of the invention, wherein the frequency controlling means adjusts a direction of an optical axis of the light reflected from the reflector by changing the frequency of the alternating current in such a manner that the direction of the optical axis of the reflected light is directed in at least two directions.
By employing these configurations, it is possible to construct a low-cost high intensity discharge lamp system having a simplified configuration that can be suitably used for, for example, vehicle headlamps in which the adjustment of optical axis or the selecting between high beam and low beam is required. Specifically, the configurations can eliminate a mechanism for adjusting an optical axis mechanically, and achieve a system in which only one lamp is necessary for both low beam and high beam. It is noted that, in addition to making the system capable of selecting a plurality of beams, such as high beam and low beam, the adjustment of the optical axis can be adapted to one of the beams, or all of the beams.